Laser propulsive transfer imaging has been studied for over 20 years. Work in this field was largely based on the use of high power flashlamp water-cooled Nd:YAG lasers capable of producing more than 5 W of power. Recently, diode-pumped solid state lasers have become available in the 0.2 to 4 W range. This laser technology would make laser propulsive transfer imaging more commercially feasible since diode-pumped lasers are compact, air-cooled, and relatively maintenance-free.
The process in which the article of the invention is used provides a donor element which has a laser propulsive transfer material, an absorber component and the material to be transferred, the latter two of which may be incorporated into a single or multilayer coating that is applied to a transparent substrate such as polyester. This donor sheet is then placed in contact with a receiver substrate (plain paper, aluminum, coated polyester, etc.) and imaged (irradiated from the back or front) with the laser. Material is transferred from the donor to the receptor only in those locations where laser heating has occurred. It is believed that the rapid absorption of laser energy produces a rapid expansion or devolution of gases in the donor sheet from thermal expansion and/or decomposition, and this expansion induces a rapid evolution of gas which has been compared to a shock wave that propels the transfer material from the donor to the receptor. Since the material is heated adiabatically, the exposure energy required is reduced to less than 0.2 J/cm.sup.2. The transfer process is fast, requiring pixel dwell times of only a few 100 ns. This means that A 3 size format images can be produced in less than 2 minutes using a 4 W laser.
In the past, carbon black/nitrocellulose coatings were used to transfer crosslinkable resins to aluminum printing plates and to make films and black and white proofs. More recently, a decomposable polymer was disclosed in U.S. Pat. Nos. 5,156,938 and 5,171,650 which could be used to transfer pigment for color proofing applications. These patents describe the use of Cyasorb 165 IR dye to absorb the laser power. This IR dye has a low absorptivity in the visible region, thus preventing excessive visible staining of the pigment. This IR (Infra-Red) dye was also used as an absorber in glycidyl azide polymer (GAP) imaging materials described in U.S. patent application Ser. No. 07/977,215 filed on Nov. 17, 1992 titled "PROPELLANT-CONTAINING THERMAL TRANSFER DONOR ELEMENTS." However, some visible residue may still be present after imaging. In addition, dye lifetime stability may also be poor.